The Ross-type of Stirling cycle machine is a well known type of kinematic (mechanically constrained motion) Stirling cycle machine. The Ross machine is commonly considered for numerous applications ranging from stationary ground-based applications to cryocooling of infrared sensors for use in outer space missions. The advantages of the Ross-type machine is that it has a mechanically simple mechanism which provides for the correct phase relationship of the piston motions for the Stirling cycle, while at the same time nearly eliminating piston side loads.
The avoidance of piston side loads reduces frictional losses and increases the life of the cylinder sliding surfaces. This is particularly important in allowing for the use of a dry sump where the crankcase does not contain an oil reservoir. With low piston side loads, teflon-based sliding surfaces can be used in the cylinders, and mechanism pivots can consist of sealed permanently-lubricated bearings. The dry sump avoids the possibility of oil entering the working space and contaminating the heat exchangers, and in the case of an air engine avoids the dangerous possibility of an explosion should oil reach the hot parts of the engine. A typical drawing of a Ross-type machine, incorporating the new balance technique, is shown in FIG. 1. The basic arrangement of the Ross-type machine utilizes separate parallel cylinders and a single throw crankshaft. The throw of the crankshaft is pivotally connected to a yoke.
Two additional pivotal connections exist on the yoke for connections to the piston driving links. A fourth pivotal connection on the yoke connects to a swinglink, the other end of said swinglink being pivotally connected to the block of the engine. Piston side loads are small because the connections on the yoke which drive the pistons approximate straight line motions.
The prior art for balancing of the Ross machine presented two options.
The first and simpler of the two, utilized a single counter-rotating balance shaft placed directly above the crankshaft of the engine. Both this balance shaft and the engine crankshaft contained balance masses positioned to lead the throw of the crankshaft by 180 degrees of rotation.
This method however required that the piston masses be made equal and also placed geometrical restrictions on the design of the mechanism.
The second prior art balancing scheme did not have the restrictions of the method described above, but required two balance shafts Because of the complication of the second balance shaft, and the fact that its inclusion also increased the mass of the machine, this second method was seldom utilized in practice.
The requirement for equal piston masses in the first mentioned balance method added additional mass to the machine. The form of the two pistons of a Stirling machine is not the same. As is seen in FIG. 1, the expansion piston 12 is more complicated than the compression piston 15, primarily because it uses a Heylandt crown 13.
The purpose of the Heylandt crown is to isolate the extreme temperature of the expansion space gas (hot gas in the case of an engine) from that of the expansion space piston seal 16. The compression piston 15 does not need a Heylandt crown and thus can be lighter than the expansion piston.
Also, because the drive point 17 on the yoke approximates straight line motion, the compression piston 15 is often constructed as shown in FIG. 1 without the wrist pin and bearing 18 used on the expansion piston.
In this case the compression piston will rock slightly during its up and down travel due to the small lateral movements of the drive point 17. The absence of the wrist pin and bearing on the compression piston also leads to the compression piston being lighter than the expansion piston.
It is sometimes possible to eliminate the expansion piston wrist pin 18, but this is less common in practice because the gap between the Heylandt crown and the cylinder would have to be enlarged to allow for rocking of the expansion piston. Enlarging this gap is often detrimental to engine performance because it increases the unswept volume of the working space and leads to increased heat transfer losses in the gap.
Thus, the mass of the compression piston can be made considerably lighter than that of the expansion piston. However, the prior art for balancing utilizing a single balance shaft required that the mass of compression piston be artificially increased to match that of the expansion piston. Also, the mass increase to the machine was not limited to that associated with the added piston mass. Many other factors resulting from the added piston mass contributed to added machine mass and cost.
The added piston mass increased the required balance forces which was often accomplished with heavier counterweights. Also many mechanism loads as well as the transmission loads between the crankshaft and balance shaft increased. This required the use of larger bearings and stronger parts which also added to the weight and cost of the machine.
This prior art balancing technique also placed restrictions on the mechanism geometry. This restriction was that the horizontal yoke arm lengths between the swinglink pivot and the piston driving points must be equal. This limited the freedom of the designer to select the most ideal arm lengths for the given machine layout.